EP0622810A1 - Method and installation for repairing and preventing cracking of the inner surface of a vessel bottom penetration pipe of a pressurized water nuclear reactor - Google Patents

Abstract

In a stoppage period of the nuclear reactor, the head of the vessel having been removed and the vessel (3) and the pool of the reactor having been filled with water, an internal zone of the penetration tube (6), for the bottom head of the vessel (3), is isolated between two seals (39, 42), this operation being carried out from the upper level of the pool and under water. An electrolyte is made to flow into the isolated zone inside the penetration tube (6) and a current is made to pass through the electrolyte in order to deposit a metal layer of electroplating on the internal surface of the penetration tube (6) in the isolated zone. The metal layer is preferably a nickel layer having a thickness of approximately 0.1 mm. The device comprises a hollow electroplating rod (cane) (25) carried by a pole (14) and connected via a set of pipes and cables (24) to an electroplating module arranged at the upper level of the reactor pool. <IMAGE>

Description

The invention relates to a method and a device for repairing and protecting against cracking the internal wall of a tube passing through the bottom of the vessel of a pressurized water nuclear reactor.

Pressurized water reactors include a generally cylindrical tank which contains the reactor core and which is arranged with its vertical axis in a tank well opening at its upper end into a swimming pool.

The tank has a closure cover at its upper part and a curved bottom at its lower part which is crossed by a plurality of substantially vertical tubes called bottom penetrations.

These tank bottom penetrations have a projecting end inside the reactor tank and an opposite end projecting under the curved bottom which is connected to a flexible measurement conduit making it possible to join the bottom of the tank to a bathroom. instrumentation arranged in the structure of the reactor building, generally in a position adjacent to the vessel well.

Each of the measuring conduits and the corresponding penetration of the bottom of the tank ensures the passage of a thermowell in which a measuring probe moves fixed to the end of a flexible element of great length and its introduction inside. of the vessel and of the core for carrying out measurements, for example neutron flux or temperature measurements, inside the core during the operation of the reactor.

The tank bottom crossing tubes are generally made of nickel alloy and are fixed by welding to the curved bottom of the tank which is generally made of high-strength steel.

The internal surface of the tank bottom penetrations is in contact, during the operation of the reactor, with the primary fluid of the reactor consisting of pressurized water containing various additives.

There has been a tendency to crack the inner surface of the tank bottom penetrations, particularly in the vicinity of the area in which the feed-through tube is welded to the tank bottom.

It may be necessary to carry out repairs to the internal surface of the penetrations, after a certain time of operation of the reactor and it is in any case advantageous to treat this internal surface preventively, to avoid or delay the cracking of the internal wall of the penetrations during reactor operation.

In the case of tubular elements which may be subjected to stress corrosion and cracking in contact with the primary fluid of a reactor, for example in the case of the steam generator tubes of pressurized water nuclear reactors, repair methods or preventive treatment methods have been provided for the internal wall of the tubular elements which make it possible to extend their service life and improve the safety of the nuclear reactor.

It has thus been envisaged, in the case of steam generator tubes which comprise a transition zone between a deformed part and an undeformed part, in the vicinity of the upper face of the tubular plate in which these tubes are fixed, a treatment of their internal wall, in the transition zone, for example by mechanical expansion, thermal stress relieving or hammering by small balls (micro-balling).

In the event that cracks have appeared, repair can also be carried out by sleeving the tube in the transition zone.

Patent applications FR-A-2 565 323, FR-A-2 585 817, FR-A-2 615 207 and FR-A-2 652 191 have also been proposed by FRAMATOME, methods and devices for depositing metal by electrolysis on the inside surface of the steam generator tubes in the transition zone. This treatment makes it possible to avoid contact between the primary cooling water of the reactor and the transition zone sensitive to stress corrosion and cracking.

The metal deposited by electrolysis is generally pure nickel, the steam generator tubes being made of a nickel-based alloy containing a proportion of the order of 75% nickel.

However, hitherto no method has been known which makes it possible to effect repair and protection against cracking of the internal wall of bottom penetrations of a nuclear reactor, after a certain time of operation of the reactor.

The invention is therefore to propose a method of repairing and protecting against cracking the internal wall of a tube passing through the bottom of a vessel of a pressurized water nuclear reactor, the vessel of generally cylindrical shape being arranged with its vertical axis in a vessel well opening at its upper end into a swimming pool and comprising a cover and a curved bottom traversed by a plurality of substantially vertical tubes each connected to an instrumentation room of the reactor, by a measurement conduit ensuring the passage of a measurement probe in the reactor core which is arranged inside the tank, this process being able to be implemented either for carrying out repairs, or as a preventive measure, and without exposing the personnel in charge of intervention with radiation from the reactor vessel.

• l'isolation d'une zone interne du tube ayant une longueur et une position définie dans la direction axiale du tube,
• le remplissage par un électrolyte de la zone isolée, et
• le passage d'un courant d'électrolyse à travers l'électrolyte pour déposer une couche métallique de revêtement électrolytique sur la surface interne du tube dans la zone isolée.

For this purpose, during a period of shutdown of the nuclear reactor, the cover of the tank and the lower and upper internal equipment of the tank being dismantled and the tank and the swimming pool being filled with water, the upper level of the pool and underwater,

• the insulation of an internal zone of the tube having a length and a position defined in the axial direction of the tube,
• filling the isolated area with an electrolyte, and
• passing an electrolysis current through the electrolyte to deposit a metal layer of electrolytic coating on the internal surface of the tube in the insulated area.

The invention also relates to a device for carrying out the implementation of the method.

• La figure 1 est une vue en perspective éclatée d'une partie du bâtiment d'un réacteur nucléaire à eau sous pression.
• La figure 2 est une vue en coupe à grande échelle d'une traversée de fond de cuve du réacteur nucléaire représenté sur la figure 1.
• La figure 3 est une vue schématique en élévation et en coupe verticale d'une installation pour la mise en oeuvre du procédé suivant l'invention.
• La figure 4 est une vue à plus grande échelle d'une partie du dispositif représenté sur la figure 3.
• La figure 5 est une vue plus détaillée en élévation et en coupe d'une partie du dispositif représenté sur la figure 4.
• Les figures 6A à 6E sont des vues schématiques montrant le dispositif de revêtement électrolytique pendant des phases successives d'une opération de revêtement à l'intérieur d'une traversée de fond de cuve.

We will now describe, by way of nonlimiting example, with reference to the appended figures, an example of implementation of the method according to the invention and the devices used for this implementation.

• Figure 1 is an exploded perspective view of part of the building of a pressurized water nuclear reactor.
• FIG. 2 is a sectional view on a large scale of a bottom crossing of the nuclear reactor shown in FIG. 1.
• Figure 3 is a schematic elevational view in vertical section of an installation for implementing the method according to the invention.
• FIG. 4 is an enlarged view of part of the device shown in FIG. 3.
• FIG. 5 is a more detailed view in elevation and in section of a part of the device shown in FIG. 4.
• FIGS. 6A to 6E are schematic views showing the electrolytic coating device during successive phases of a coating operation inside a bottom tank crossing.

In FIG. 1, we see a part of the concrete structure 1 of a nuclear reactor delimiting a tank well 2 in which the tank 3 of the nuclear reactor is arranged, a swimming pool 4 in which the well 2 opens out from its upper part. and an instrumentation room 5 in an adjacent position relative to the tank well 2.

The tank 3 of generally cylindrical shape has a convex bottom 3a crossed by penetrations of the bottom of the tank 6 which are each connected, outside the tank 3, below the bottom 3a, to an instrumentation guide tube 7 ensuring a connection between the bottom 3a of the tank and the instrumentation room 5 of the reactor.

Inside each of the guide tubes 7, it is possible to introduce and move from the instrumentation room 5, a measurement probe fixed to the end of a flexible element of great length, so as to introduce the measurement inside the tank 3 and in the core 8 of the reactor arranged inside the tank 3, to carry out measurements, for example measurements of neutron flux or temperature during the operation of the reactor.

The measurement signals are transmitted by the probe which can be moved inside the heart, in the instrumentation room 5 where these measurement signals are collected and processed by measurement units.

In Figure 2, we see a part of the domed bottom 3a of the nuclear reactor vessel, at the level of a tank bottom penetration 6 which consists of a through tube engaged practically without play in an opening passing through the tank bottom and welded to the tank bottom, at its internal face, by means of a seal 9 of filler metal deposited in a chamfer into which the opening opens.

The feed-through tube 6 has a projecting part inside the nuclear reactor vessel, above the vessel bottom 3a, the end 6a of which is chamfered, in the form of a frustoconical surface.

The lower end 6b of the crossing tube 6 disposed outside the tank, below the bottom 3a, has a bore diametrically widened relative to the bore of the current part of the tube 6. This widened bore allows the mounting the guide tube 7 inside the bushing tube 6. The guide tube 7 is welded to the lower end of the tube 6.

The axis 10 of the opening and the tube 6 of the crossing has a vertical direction.

After a certain operating time of the nuclear reactor, the internal surface 6c of the tube 6 is likely to include cracks or crack initiators, mainly in the region adjacent to the weld bead 9.

Such cracks or crack initiations can be detected using an eddy current probe which is introduced into the internal bore of the bushing 6, during a period of shutdown of the nuclear reactor, the cover of the vessel being removed and the tank and pool being filled with water.

In the event that cracks are detected on the internal wall 6c of a bushing 6, the repair can be carried out using the method according to the invention.

It may also be advantageous to carry out the treatment according to the invention, on all the bushings 6 at the bottom of the tank 3, as a preventive measure.

In all cases, an intervention installation as shown in FIG. 3 can be used.

The intervention is carried out during a nuclear reactor shutdown period, the cover of the tank 3 being removed and for example placed on a reception stand on the bottom of the pool 4 which is filled with water to a level 4a, slightly below the upper edge 11 of the swimming pool.

After removing the cover, the polar bridge of the nuclear power station is used, to ensure the lifting and extraction of the vessel 3, upper internal equipment of the reactor arranged above the core, this upper internal equipment being deposited on a storage stand 12 in the reactor pool.

Next, the core assemblies are unloaded and transported to a storage pool.

The dismantling and transport of the lower internal equipment of the reactor is then carried out, which are deposited on a storage stand 12 'at the bottom of the pool.

It is then possible to access from the top, the upper ends 6a of the tank bottom crossings 6.

The installation making it possible to implement the method according to the invention, represented in FIG. 3, comprises a fixed part resting on a floor at the upper level 11 of the swimming pool and a mobile part which can be fixed for example on the machine of loading 13 of the reactor which moves on rails resting on the floor at the upper edge 11 of the swimming pool.

The mobile part of the installation comprises in particular a vertical pole 14 fixed at its upper part to the carriage of the loading machine 13 and a vertical guide duct 15 fixed along the pole 14.

The pole 14 further carries, at its lower part, in the axial extension of the guide duct 15, a guide and introduction tip 16 which can be placed by the pole 14, so as to come to rest on the part of upper end 6a of any tank bottom penetration 6 passing through the bottom 3a of the tank 3.

By moving the carriage of the loading machine 13 and the pole which is fixed to a handling assembly resting on the carriage of the loading machine 13, penetration 6 can be achieved in any position on the bottom 3a of the tank 3.

The device as shown can firstly be used to carry out the control of penetrations, by introducing a control device with eddy current probe inside the bore of the penetration 6 via the conduit 15 and tip 16.

The information given by the eddy current probe is processed at a control and command station 18 constituting a fixed part of the installation resting on the floor at the upper edge 11 of the swimming pool 4.

The fixed part of the installation comprises, in addition to the control and command station 18, a group 19 of electrical supply, four pumping and heating groups 20 of the products used for the electrolytic coating and a control unit 21 bringing together the control valves of the electrolytic coating device.

The installation also includes a reel 22 carried by the carriage 13 of the loading machine and a cable accumulator 23, the reel 22 and the accumulator 23 making it possible to move a very long assembly 24 comprising a tubular or umbilical envelope containing conduits and cables for supply and measurement and a traction cable ensuring the displacement of the navel which is connected at its end to an electrolytic coating rod 25.

The installation shown in Figure 3 also includes a control station 26 resting on the floor at the upper level 11 of the pool and can be submerged at the bottom of the tank or the pool.

As can be seen in FIG. 4, the umbilicus 24 is connected at its lower part to an electrolysis rod 25 and at its opposite end, at the upper edge 11 of the swimming pool, to an electrolytic coating module 30 bringing together the various groups and units 18, 19, 20 and 21 shown in FIG. 3.

The module 30 makes it possible, via the navel 24, to supply the rod 25, with electrolyte liquid, with fluid for controlling the seals and with electrolysis current, in particular.

When the rod 25 is raised inside the guide tube 15, the additional length 24 ′ of the navel 24 is stored by the accumulator device 23, as shown diagrammatically in dotted lines.

As can be seen in FIG. 5, the pole 14 carries the electrolysis rod 25, by means of a plate 27 which is slidably mounted in the axial direction, on the lower part of the pole 14 and which can be moved in the vertical direction in one direction and in the other by a motorization 28 of raising and lowering of the plate 27.

The guide duct 15 is fixed to the pole 14, in an arrangement parallel to the latter, by support means such as 29 distributed along the length of the pole.

The navel 24 passes inside the guide duct 15 in which it is arranged in the axial direction. A traction cable 31 is attached to the navel 24, so as to ensure the ascent of the navel inside the guide duct 15, during the ascent of the rod to the upper level of the pool, by example to ensure the change of the rod electrode 25.

As can be seen in the upper part of the rod 25 in FIG. 5, the umbilicus 24 ensures the passage of a set of cables and supply lines 32 comprising an electrical supply cable for the electrode, the pressurized nitrogen supply ducts for tightening cylinders of the cane seals and the supply and circulation ducts for the electrolyte liquid.

The cane 25 is produced in a tubular form and has different sections connected end to end, along the length of the cane.

A first section 33 is constituted by a flexible conduit on which is engaged and externally fixed an adjustable stop 34 provided with an annular seal 35 on its lower part and which is connected at its upper part to a locking piece 36 allowing the fixing of the rod 25 on the movable plate 27.

The upper part of the rod 25 is connected, at its upper part, above the locking part 36, to the navel 24 allowing the electrical connection and the connection of the fluid conduits to different channels passing through the rod 25 over its entire length.

The flexible conduit 33 of the rod 25 is connected, at its lower part, to a connection and sealing assembly 37 comprising a radially expandable annular seal 39. The connection and sealing assembly 37 is itself connected to the electrode 40 of the rod 25 formed in the form of a tubular metal part connected at its lower end to a closure and sealing assembly 41 comprising a seal 42 which is radially expandable and terminated by a warhead.

The sealing assemblies 37 and 41 comprise two clamping parts between which the seals 39 and 42 are interposed respectively.

The clamping parts of the seals 39 and 42 are movable relative to each other in the axial direction and allow the clamping in the axial direction and the radial expansion of the seals 39 and 42 so as to ensure the sealing in the bore of the tank bottom bushing.

The clamping parts of the seals 39 and 42 can be controlled in the clamping direction by a device constituting a pneumatic cylinder supplied with compressed nitrogen by pipes passing through the umbilicus 24 and in the direction of loosening, by helical springs.

The expansion and radial retraction of the joints can therefore be controlled remotely, from the upper level of the swimming pool, via the control station 18 and the control unit 21 comprising solenoid valves inserted on a nitrogen circuit. under pressure.

The guide and introduction endpiece 16 of the rod 14 is fixed to the end of the rod, by means of a support 43.

The endpiece 16 has a substantially frustoconical engagement opening at its packed bottom internally with a seal 16a intended to come into contact with the upper part 6a of the penetration of the bottom of the tank 6, during the positioning of the electrolytic coating rod and an upper flared inlet end.

The electrode 40 of tubular shape of the rod 25 is pierced with electrolyte fluid inlet and outlet openings in its side wall, these openings being connected to internal annular conduits of the cane 25 connected to the electrolyte circuit comprising conduits passing inside the navel 24.

The metallic tubular piece constituting the external part of the electrode 40 is connected to the electric supply cable, so as to establish an electric field and to circulate an electrolysis current between the electrode 40 and the internal surface of the tube. bushing 6, when the rod 25 is positioned inside the bushing tube 6 to perform an electrolytic coating operation.

The tool-holder plate 27 is mounted sliding in the axial direction on the lower part of the pole 14 and can be driven in translation in the axial direction, in one direction and in the other to effect the introduction of the rod 25 into the cross tube 6 and the withdrawal of the rod 25, respectively. These displacements can be obtained by means of a screw arranged in the longitudinal direction of the pole coming into engagement with a nut carried by the plate 27 and driven in rotation by the motorization of raising and lowering of the plate 28.

We will now refer to FIGS. 6A to 6E to describe an electrolytic coating operation inside a feed-through tube 6 at the bottom of the tank 3a.

Prior to the actual electrolytic coating operation, the interior surface of the bushing tube 6 is brushed in the area in which the repair and / or protection treatment is to be carried out, this area generally being close to the weld 9 of the crossing tube on the bottom of the tank 3a and having an axial length of the order of 150 mm. Brushing is carried out on the inner surface of the through tube 6, in an area of 160 mm length entirely covering the area of length 150 mm in which the repair or protection treatment is to be carried out.

To carry out this brushing operation, a motorized tool is used comprising a cylindrical brush fixed on the axis of a motor which can be put in place either by using the pole 14, the guide duct 15 and the nozzle 16, or an independent device, which can be placed on the upper part 6a of the feed-through tube 6, using a lifting and handling means of the loading machine or the bridge of the nuclear reactor.

When sufficient brushing has been carried out inside the crossing tube 6, in the area to be treated, the brushing tool is extracted from the crossing 6 and brought back to the upper level of the swimming pool.

Then put in place in the pool the pole 14, as shown in Figure 6A, so that the guide and introduction nozzle 16 is located above and vertically above the cross tube 6 of the bottom of tank 3a.

A welding rod 25 comprising the flexible conduit 33 and the electrode 40 is connected to the end of the navel 24. The cables and conduits passing through the navel 24 are connected to the rod 25 so as to supply the electrode with electric current and electrolyte.

As can be seen in FIG. 6B, the fitting 16 is then put in place on the end 6a of the crossing tube 6, by lowering the pole 14 by the desired height. The insertion and guide endpiece 16 comes into contact with the upper end 6a of the feed-through tube by means of the seal 16a disposed inside its lower flared part.

The position of the stop 34 is adjusted to the desired height, so as to introduce the electrode 40 into the part of the tube 6 where the treatment is to be carried out.

The rod 25 is fixed by means of the locking piece 36 to the plate 27.

The motorization 28 of the plate 27 is put into operation in the direction of the descent of the plate 27 which drives the rod 25 downwards inside the insertion and guide nozzle 16 comprising an upper flared end, then inside the bore of the through tube 6, as shown in Figure 6C.

At the end of the cane 25 lowering movement, the stop 34 comes to rest on the upper end of the crossing tube 6, by means of its seal 35.

The introduction of the rod 25 inside the crossing tube 6 is facilitated by the fact that the rod 25 has a flexible part 33 capable of adapting, in the case of a defect in orientation of the crossing tube 6, relative to the vertical direction.

The plate 27 firmly holds the stop 34 against the end of the through tube 6, so that the seal 35 ensures a tight closure of the annular space between the rod 25 and the bore of the feed-through tube 6. The upper part of the duct 33 between the locking piece 36 and the stop 34 has sufficient rigidity to ensure effective support of the seal 35.

Then pressurized nitrogen is sent into the electrolyte supply circuit of the rod 25 which is ejected through the openings of the electrode 40 inside the feed-through tube 6. The pressurized nitrogen ensures the blowing the water contained in the crossing tube 6 and in the corresponding guide tube 7 opening into the instrumentation room 5. The water expelled from the crossing tube 6 and the guide tube 7 is collected in the room instrumentation and poured into a container for collecting liquid effluents from the plant.

As shown in FIG. 6D, the axial tightening is then carried out causing a radial expansion of the seals 39 and 42 by means of the corresponding tightening assemblies 37 and 41. This thus isolates an annular zone around the electrode 40 over the entire length of the area to be treated, in the vicinity of the weld 9 of the tube 6. This area, the position of which is perfectly adjusted by means of the stop 34, has a length between the seals 39 and 42, of the order of 150 mm.

The electrolyte circulation circuit of the rod 25 is then supplied with an electrolyte whose active product consists of nickel sulfamate, in the case of the treatment of a nickel alloy bushing and which is heated to a temperature of the order of 50 to 70 ° C. The electrode 40 is supplied with electrolysis current, so as to deposit a layer of nickel on the inner surface of the feed-through tube 6, between the seals 39 and 42.

The electrolysis parameters are set prior to the coating operation, at the control station 26.

It is possible to operate in several stages, with a circulation of different electrolytic baths during a first stage of producing a preliminary layer and a second stage of depositing the protective nickel layer, the two stages being separated by rinsing with water from the inner surface of the tube in the coating area.

Generally, in the case of tank bottom bushings with an inside diameter of around 15mm, a protective nickel layer with a thickness of around 0 is deposited on the inside surface of the bushing, 1 mm.

This deposition is carried out in a period slightly greater than 20 min.

The seals are then loosened by releasing the nitrogen pressure in the circuits of the seals tightening cylinders.

The clamping parts are moved away from each other under the effect of the return springs. The joints which are no longer compressed in the axial direction return to their initial diameter which is less than the inside diameter of the bushing.

The device has returned to its configuration shown in FIG. 6C.

Water is then sent under pressure into the electrolyte circulation circuit, so that the water is ejected through the openings passing through the electrode 40, into the bore of the bushing 6. The rinsing is thus carried out of the treated area, of the bore of the tube 6 and of the guide tube 7.

The rinsing water is collected in the instrumentation room.

It then controls the movement of the plate 27 upward, so as to raise the rod 25 into its position shown in Figure 6B.

As shown in FIG. 6E, the rod 25 carried by the pole 14 can be moved to come into line with a bushing 6 ′ different from the bushing 6, in which an electrolytic coating treatment operation must be carried out.

The layer of nickel deposited on the interior surface of the crossing tube 6 made it possible to seal the cracks or crack initiators possibly present on this surface, in the treated area which is thus repaired.

In addition, the nickel layer makes it possible to isolate the interior surface of the bushing tube, so that after restarting the reactor, the interior surface of the bushing no longer comes into contact with the pressurized water for cooling the reactor. , in the paved area.

A protective nickel layer can be deposited, as a preventive measure, in all of the tubes passing through the bottom 3a of the tank, in the zone most likely to undergo cracking, that is to say in the zone of the welding of the tubes on the bottom of the tank.

Such a preventive treatment makes it possible to avoid the appearance or the development of cracks in the areas close to the welds of the crossing tubes, during the operation of the reactor.

The nickel layer deposited on the interior surface of the feed-through tube intended to come into contact with the pressurized water of the reactor is free from defects and stresses, so that it is practically insensitive to corrosion by the primary fluid of the reactor.

The method according to the invention therefore makes it possible to substitute for a layer of metal having constraints and defects created by the welding of the tubes of crossing, a virgin layer free of defects, produced by electrolysis.

In addition, the electrolytic nickel layer resists abrasion due to the friction of the thermowell during extraction or replacement of the probe in the heart.

The method of the invention therefore makes it possible to carry out an effective preventive repair or treatment inside the tubes passing through the bottom of a vessel of a pressurized water nuclear reactor, so as to increase the safety of the nuclear reactor. and extend its lifespan.

The invention is not limited to the embodiment which has been described.

This is how deposition by electrolysis of a metal other than nickel can be carried out, in the case of feed-through tubes which would consist of an alloy different from a nickel alloy.

In all cases, a metal will be chosen which is compatible with the metal of the feed-through tube and which can be deposited in the form of a homogeneous layer, by an electrolysis treatment.

It is quite obvious that it is possible to carry out a preliminary treatment of the inner surface of the tube, before the electrolytic coating, this preliminary treatment possibly being brushing, machining, pickling or electrolytic polishing or any combination of these treatments.

The invention can be implemented by using means different from those which have been described, in particular by using a rod having a structure and for example means for inflating the joints different from those which have been described.

The treatment can be carried out without prior blowing of a neutral gas to expel the water present in the crossing tube, the tightening of the joints being carried out immediately after the introduction of the cane into the crossing tube filled with water. The water contained in the treatment space between the seals is then evacuated by the electrolyte circulation circuit.

The method and the device of the invention apply to the repair or protection of any tube passing through the bottom of the vessel of a nuclear reactor cooled by pressurized water.

Claims (14)

1.- A method of repairing and protecting against cracking the internal wall of a tube (6) for crossing the bottom (3a) of a vessel (3) of a pressurized water nuclear reactor, the vessel ( 3) of generally cylindrical shape being disposed with its vertical axis in a tank well (2) opening at its upper end into a swimming pool (4) and comprising a cover and a curved bottom (3a) crossed by a plurality of tubes (6 ) substantially vertical, each connected to an instrumentation room (5) of the reactor, by a measurement conduit (7) ensuring the passage of a measurement probe inside the core (8) of the reactor which is disposed in the tank (3), characterized in that, during a nuclear reactor shutdown period, the tank cover (3) as well as the upper and lower internal equipment of the tank being removed and the tank (3) and the pool (4) being filled with water, we realize from the upper level (11) from the swimming pool (4) and underwater, - the insulation of an internal zone of the tube (6) having a length and a position defined in the axial direction of the tube (6), - filling the isolated area with an electrolyte, and - the passage of an electrolysis current through the electrolyte to deposit a metal layer of electrolytic coating on the internal surface of the tube (6) in the insulated area. 2.- Method according to claim 1, characterized in that the electrolyte is circulated in the isolated area inside the crossing tube (6). 3.- Method according to any one of claims 1 and 2, for the coating of a nickel alloy pass-through tube, characterized in that the metallic layer of electrolytic coating on the internal surface of the feed-through tube (6) consists of nickel. 4.- Method according to claim 3, characterized in that the active product of the electrolyte consists of nickel sulfamate. 5. A method according to any one of claims 3 and 4, characterized in that the electrolytic coating is continued for a sufficient time to obtain a metallic layer of nickel coating on the internal surface of the through tube (6), about 0.1 mm thick. 6.- Method according to any one of claims 1 to 5, characterized in that the internal surface of the crossing tube (6) in the isolated area comprises at least one crack which is repaired by electrolytic coating. 7.- Method according to any one of claims 1 to 5, characterized in that the electrolytic coating is produced on the internal surface of each of the tubes (6) passing through a tank bottom (3a), successively, preventively. 8.- Method according to any one of claims 1 to 7, characterized in that before carrying out the insulation of the internal zone of the crossing tube (6), a blowing is carried out with an inert gas inside of the bushing tube (6), in order to expel the water in the bore of the tube (6). 9.- Method according to any one of claims 1 to 8, characterized in that before carrying out an electrolytic coating on the internal surface of the through tube (6), a brushing of the internal surface of the tube is carried out. bushing (6), in the area intended to receive the electrolytic coating. 10.- Device for repairing and protecting against cracking the internal wall of a through tube (6) at the bottom (3a) of a tank (3) of a pressurized water nuclear reactor, the tank ( 3) of generally cylindrical shape being disposed with its vertical axis in a tank well (2) opening at its upper end into a swimming pool (4) and comprising a cover and a curved bottom (3a) crossed by a plurality of tubes (6 ) substantially vertical, each connected to an instrumentation room (5) of the reactor, by a measurement conduit (7) ensuring the passage of a measurement probe inside the reactor core which is disposed in the vessel (3 ), characterized by the fact that it comprises: a means (13) of mobile lifting on a horizontal platform above the upper level (11) of the pool (4) of the reactor, - a long pole (14) fixed by its upper part, in a vertical arrangement, on the lifting and handling means (13), - a tip (16) for introduction and guide fixed to the lower end of the pole (14), - a guide duct (15) fixed along the length of the pole (14), in the extension and above the end piece (16), - an electrolytic coating rod (25) connected via a set of conduits and supply cables (24) to an electrolysis module (30) comprising electrolyte pumping groups (20) and power supply means (19) for supplying the rod (25) which comprise a tubular electrode (40) interposed between two radially expandable seals (39, 42) associated with controlled clamping devices remotely via a fluid circuit, and - A plate (27) for fixing and moving the rod (25) movable in the vertical longitudinal direction of the pole (14). 11.- Device according to claim 10, characterized in that the electrolysis rod (25) comprises a rigid part comprising the electrode (40) and a flexible part (33) connecting the electrode (40) to the conduit ( 24) supply of fluid and electric current. 12.- Device according to claim 11, characterized in that an adjustment stop (34) of the position of the electrode (40) inside the traversing tube (6) is fixed in an adjustable manner in the axial direction on the flexible part (33) of the electrolysis rod (25), so as to come to rest on an upper end part (6a) of the bushing (6) during the positioning of the rod ( 25) inside the tube (6). 13.- Device according to claim 12, characterized in that the stop (34) comprises a seal (35), the stop (34) coming to bear on the end of the crossing tube (6) by l 'through the seal (35). 14.- Device according to any one of claims 10 to 13, characterized in that it further comprises a winding device (22) and an accumulator device (23) of the supply conduit (24) fixed to the rod electrolysis (25), the reel (22) and the accumulator (23) being fixed on the lifting and handling means (13) circulating at the upper level (11) of the swimming pool (4).

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